1. Technical Field
The present invention relates to power amplifiers and, more particularly, to a Class-D power amplifier having the function of distortion suppression.
2. Description of Related Art
Power amplifiers are essential for driving loudspeakers. Common power amplifiers include Class-AB power amplifiers and Class-D power amplifiers. Therein, a Class-D power amplifier is superior to a Class-AB one in efficiency by two to five folds. Besides, Class-D power amplifiers have significantly lower energy demand and power consumption than Class-AB power amplifiers.
It is known that a Class-D power amplifier implements the PWM (Pulse Width Modulation) technology to covert input audio signals into switch signals, which turn on or off power electronic devices so as to amplify the audio signals. However, the amplified audio signals output by the Class-D power amplifier also contain modulated high-frequency harmonics so that a low-pass filter has to be provided at the output end at the rear stage of the Class-D power amplifier to filter out such high-frequency harmonic components. Then, after waveform restoration, the audio signals are delivered to a loudspeaker for playing. Nevertheless, the low-pass filter is typically realized by an LC filter, which increases the volume and circuit costs of the resultant Class-D power amplifier.
Therefore, research and development of filterless Class-D power amplifiers have been carried out. Please refer to FIG. 1, which is a circuit diagram of a conventional filterless Class-D power amplifier. The depicted filterless Class-D power amplifier 500 includes an input amplifier 502, an error amplification unit 503, a first reference wave generator 504, a second reference wave generator 505, a first comparator 506, a second comparator 507, a gate drive circuit 508, an output stage FET (Field Effect Transistor) unit 509, and a feedback unit 513.
An audio signal 501 is input to the negative input end of the input amplifier 502 through an input capacitor CIN5. The input amplifier 502 pre-amplifies the audio signal 501 and inputs a pre-amplified audio signal to the error amplification unit 503, i.e., the negative input end of an integrator 5031, through an integrator resistor R51. Therein, the integrator 5031 is coupled with the integrator resistor R51 and an integrator capacitor C51. The pre-amplified audio signal is amplified and sent by the error amplification unit 503 to the positive input ends of the first and second comparators 506, 507. The negative input ends of the first and second comparators 506, 507 are respectively coupled with the first and second reference wave generators 504, 505, which generate a first reference wave and a second reference wave, respectively. By comparing the audio signal with the reference wave, each of the first and second comparators 506, 507 outputs a PWM signal to the gate drive circuit 508 so that the gate drive circuit 508 is driven to turn on or off the output stage FET unit 509, which is typically composed of power transistors. Consequently, the audio signals are output from the positive output end OUTP and the negative output end OUTN of the filterless Class-D power amplifier 500 to a loudspeaker 512. Meantime, a feedback amplifier 5131 and feedback resistors RFB1, RFB2, RFB3 are used for feedback control of the audio gain.
The aforementioned filterless Class-D power amplifier, though effective in outputting audio signals to loudspeakers and responding to input audio signals by means of the feedback unit, has its defects and shortcomings.
Firstly, the feedback unit is incapable of clamping the audio gain. While the feedback unit positively responds to the input audio signal, it fails to prevent audio distortion caused by excessive audio gain. FIG. 2 provides output waveforms of the conventional filterless Class-D power amplifier. In the waveform diagram, the double-headed arrows indicate intervals where the positive output end OUTP outputs a high-level signal while the negative output end OUTN outputs a low-level signal, or intervals where the negative output end OUTN outputs a high-level signal while the positive output end OUTP outputs a low-level signal. Since the audio signal received by the loudspeaker is the difference between the signals output from the positive and negative output ends, when the difference results from subtracting a relatively low-level audio signal from a relatively high-level audio signal, the resultant audio signal that is received by the loudspeaker tends to be distorted due to excessive gain.
On the other hand, the two-stage audio signal amplification process carried out by the input amplifier and the error amplification unit is very complicated and may lead to gate delay when signals are output from the front stage to the rear stage, thus incurring audio distortion.
With the ambition to remedy the defects of the conventional filterless Class-D power amplifiers, the inventor of the present invention has given every effort to research and innovation, and finally develops a novel Class-D power amplifier having a distortion-suppressing function.